The architecture of airplanes is evolving at present toward a broader use of electric energy. The need for a system of conversion and storage of energy is therefore also in the process of evolving due to the electrification of systems customarily utilizing pneumatic or hydraulic energy. The advent of new highly critical applications requiring operation from a normal and/or backup electrical source makes the structure of the electrical networks more complex. These new applications impose new constraints which are hard to reconcile with the current normal and backup electrical networks.
High voltage direct current networks have been implemented aboard modern airplanes. One voltage commonly used is 540 V dc. Voltages of 350 V dc and 270 V dc are also being contemplated. These networks are known as HVDC from their abbreviation: High Voltage Direct Current.
Electrical actuators are being used increasingly, especially for the landing gear brakes or flight controls. Among these, one finds notably electromechanical actuators, or EMAs, electrohydrostatic actuators or EHAs, and electrical backup hydraulic actuators or EBHAs. These actuators are generally powered by a high voltage direct current network HVDC. Furthermore, other types of loads, notably avionic computers, generally make use of a low voltage direct current network. Thus, one feels a need for a hybridization of the types of networks, both in normal and backup operation.
The use of energy storage in the form of 28 V dc batteries is conventional for the electrical networks of airplanes. In normal operation, the batteries are charged by a low voltage direct current or LVDC network, and in backup mode one draws energy here to power the backup networks. The backup low voltage direct current network draws its energy directly from a battery, while a dedicated step-up converter makes it possible to power the high voltage direct current network from a battery. The multiplication of dedicated converters for each system represents a development/maintenance cost and a significant weight. The adoption of advanced conversion techniques such as interleaving or soft switching makes it possible to limit the weight and volume of these networks. However, the costs and weight of these networks remain elevated.
At present, a converter in normal operation is associated with a load. To ensure the backup operation, a second converter is usually added to power the same load. For example, the braking system (or the flight controls) is powered in normal operation directly by the HVDC networks. In backup braking duty or in cases when the main high voltage alternating current or HVAC network is not available, specific backup converters are used to convert the energy coming from the 28 V dc battery and create an HVDC voltage. The braking system is known by the name EBAC for Electrical Brake Actuation Controller.
Likewise, the starting system of the auxiliary power unit or APU is powered in normal operation by a main HVAC network. In the absence of the HVAC network, the starting system of the APU is powered by a LVDC network via a specific LVDC/HVDC step-up converter.
The association of a specific converter for each of the conversion functions associated with the normal and backup electrical brakes, the starting of the APU on battery, the powering of the 28 V dc loads from the main HVAC network presents several drawbacks. The weight of the onboard converters is important due to the lack of optimization of the installed conversion power with regard to the instantaneous need. The weight proportion of the step-up converters is significant, representing nearly 50% of the weight of the complete system. Moreover, the converters are specific to their functions, making the costs of development and maintenance relatively elevated. The extensive use of dedicated converters in the 28 V dc networks represents an important cost and weight for these systems.
Finally, certain applications require an important availability rate which is hard to achieve with a single converter. The loss of the converter represents the loss of the associated load, which thus leads to the use of a backup converter for critical applications, further increasing the associated weight and cost.
In the 28 V dc electrical systems onboard an airplane, the backup and starting systems of the APU make use of dedicated step-up converters to create an HVDC voltage from 28 V dc batteries. These converters operate only during particular phases of flight and during relatively short periods of time. The step-up converters associated with the backup and starting system of the APU thus have a very low utilization ratio. Outside of their short periods of operation, they represent a dead weight to the airplane.
In normal operation, the main conversion system utilizes power converters to transform the main HVAC or HVDC network into regulated 28 V dc. In the case of the HVAC network, the conversion is done in two cycles, HVAC to HVDC and HVDC to 28 V dc. In backup use or when the HVAC network is down, the 28 V dc users are directly powered from batteries, leaving the main HVDC/28 V dc converters unused.
In backup operation or when the main network is down, the EBAC electrical brake or flight control systems utilize dedicated backup converters to convert the energy coming from one of the 28 V dc batteries into HVDC. In a similar manner, when the main HVAC onboard network is down, the starting system of the APU utilizes a dedicated starting converter to convert the energy coming from one of the 28 V dc batteries into HVDC.